This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Peripheral arterial disease (PAD) is a form of lower extremity atherosclerosis estimated to affect 8-12 million Americans. A progression of limb atherosclerosis may result in critical tissue ischemia, causing disabling intermittent claudication, rest pain and limb loss. The most commonly used diagnostic parameter of PAD is Ankle/Brachial index (ABI), which measures the ratio of ankle and brachial systolic pressures. It is generally agreed that an ABI<0.95 is abnormal. However, ABI does not correlate well with impairment in exercise test (e.g. walking distance). Other non-invasive diagnostic techniques include Duplex Ultrasound, Magnetic Resonance Angiographic (MRA), and Exercise Testing. A novel approach to simultaneously assess microvascular blood flow and oxygenation consumption in the ischemic muscles may allow for further understanding of the pathophysiology of PAD. In principle NIRS can continuously measure oxy-hemoglobin and deoxy-hemoglobin concentrations in deep tissues (depth up to several centimeters). Unlike Duplex Ultrasound, NIRS is sensitive to smaller vessels such as arterioles, capillaries, and venules. It provides high temporal resolution (~100 ms), relatively low spatial resolution (>1 mm) measurement, and is inexpensive and portable. In this project, a concurrent measurement of NIRS and MRI will be performed on 5 patients with PAD and 5 healthy subjects. The femoral artery was occluded and re-opened by inflating and deflating a cuff. By comparing the oxygen consumption and blood flow change during and after cuff deflation, a difference is expected between those two groups.